In my district, Google has really gone all-out to get Docs integrated into the teacher's repertoire and support it as much as possible. Every teacher, administrator, and student has a permanent account maintained by one IT person at HQ.
The collaboration is the primary driver. gDocs is way ahead of iWork and even my youngest kids are now comfortable working in Documents and Slides (1st graders making slide presentations! The mind boggles).
Being able to log in from anywhere (school and home) is the secondary driver and, again, this is a place where Apple is too little and too late.
The iPads in our elementary school are being used less and less. It's all on Chromebooks now and, yeah, they're cheap/disposable and all the other things people are saying in this thread.
But the software is the killer.